A cellular mobile radio system may comprise a plurality of cells each of which has at least one base station for simultaneous communication with a number of mobile stations or units. When the mobile stations are used for calling purposes, for example, signal transmission is effected between a base station and the mobile stations served by the base station in the cell by transmitting and receiving radio signals.
Efficient channel reuse is of particular importance in the design of large high-capacity cellular radio systems such as systems using frequency division multiple access (FDMA) or time division multiple access (TDMA) techniques. In a multi-cell environment, however, co-channel interference caused by frequency reuse is the single most limiting factor on the system capacity. Specifically, the attenuation suffered by a signal over distance is insufficient to isolate the cells from each other. One means for controlling co-channel interference which has gained increasing attention is the use of transmitter power control. The basic idea is to adjust the transmitter power in each base-mobile link so that the interference levels at other receiver locations are minimized. Maintaining sufficient transmission quality on the communication links, however, is a critical constraint. If, for example, a signal transmitted by a mobile station arrives at the base station receiver at a power level that is too low, the bit error rate may be too high to allow high quality communication. If, however, a signal transmitted by a particular mobile station arrives at the base station receiver at a power level that is too high, this high power interferes with the signals transmitted by other mobile stations that are sharing the same channel. System capacity, therefore, can be maximized if the transmitter power of each mobile station is controlled such that the transmitter signal arrives at the base station at the minimal signal to noise interference ratio which allows acceptable data recovery.
Early work on power control discovered that a good measure of quality in cellular systems design is the carrier-to-interference ratio (CIR). In an article by Aein, the concept of CIR balancing in the context of satellite systems is introduced. J. M. Aein, "Power Balancing in Systems Employing Frequency Reuse," COMSAT Tech. Rev., vol. 3, no. 2, pp. 277-300 (1973). This article and all other publications referred to herein are incorporated by reference. The power balancing approach aims at achieving the same CIR in all communication links. In another article by R. W. Nettleton and H. Alavi, "Power Control for Spread-spectrum Cellular Mobile Radio System," Proc. IEEE Vehic. Tech. Conf., VTC-83, pp. 242-246 (1983), the balancing concept is used in the context of cellular radio.
Recent work has emphasized distributed, or local, control. In a distributed power control system, the power level of each transmitter is guided, using local measurements only, so that eventually all receivers meet the specified CIR requirements. Distributed power control is of special interest because the alternative of centralized power control involves added infrastructure and network vulnerability.
Mathematical analysis in the area of distributed power control has followed two distinct paths. The first is concerned with maximizing the minimum CIR. For example, J. Zander, in "Performance of Optimum Transmitter Power Control in Cellular Radio Systems," IEEE Trans. Vehic. Tech , vol 41, no. 1, pp 57-62 (1992), presents an iterative scheme, that operates in the absence of receiver noise, to evolve the power of the signals of a specified number of users to achieve the greatest CIR that they are jointly capable of achieving. This approach, however, neglects receiver, thermal and external noise, and the vector of transmitter powers converges to within a constant of proportionality.
The second approach recognizes the presence of noise and sets as its goal the requirement that the CIR of all links not be less than some pre-fixed target, which is determined by quality of service considerations. For example, G. J. Foschini, in "A Simple Distributed Autonomous Power Control Algorithm and Its Convergence," IEEE Trans. Vehic. Tech , vol 42, no. 4, pp 641-646 (1993), provides a synchronous algorithm by which all users concurrently proceed in an iterative manner to reset their respective power levels to the level that each one needs to have acceptable performance. Each user proceeds as if the other users were not going to change their power levels. The distributed synchronous algorithm converges exponentially.
One requirement of the model discussed by Foschini is synchrony among the various users. The cost of power control in a cellular radio system, however, increases when the demands of synchrony are imposed. In order to achieve synchrony, expensive clock or timing mechanisms must be used. Alternatively, less expensive clock mechanisms may be used in conjunction with a form of feedback such as phase-locked loops to achieve synchronous functioning. In either case, the cost of obtaining power control increases as the degree of synchrony rises.
Systems which include methods for implementing power control in cellular radio systems have been proposed. U.S. Pat. No. 5,267,262 discloses a power control system for a cellular mobile telephone system using a code division multiple access (CDMA) technique. It includes means for controlling the power generated by a particular mobile unit and received at the particular base station communicating with that mobile. The mobile unit transmitted power is measured as received at the base station. The measured signal strength is compared to a desired signal strength level for that particular mobile. A power adjustment command is generated and sent to the mobile unit. In response to the base station power adjustment command, the mobile unit increases or decreases the mobile unit transmitter power by a predetermined amount, nominally less than 1 dB. Systems using CDMA, however, do not use channels. Thus, although the use of power control to balance the goals of high system capacity and high transmission quality is important, the notion of efficient channel reuse is not relevant to systems using CDMA techniques.
U.S. Pat. No. 5,241,690 also discloses a method for regulating power in a digital mobile telephony system. The output power of a mobile unit or base station is regulated to maintain the transmission power at an optimum level. The measured values of signal strength and transmission quality are collected, and their mean values are calculated. An anticipated value of signal strength and transmission quality is calculated at a future point in time. The transmission power at a future time is regulated on the basis of these anticipated values. The transmission power is increased when the anticipated transmission quality is less than that desired and is decreased when the anticipated quality is higher than the highest permitted quality or when the anticipated signal strength is greater than the maximum permitted value.